1. Field of the Invention
This invention relates to a reproducing apparatus and more particularly to a reproduction equalizing process to be performed by the apparatus on a digital signal reproduced from a recording medium.
2. Description of Related Art
In transmitting a signal, it has heretofore been practiced to obtain an adequate signal by carrying out, on the signal receiving side, an equalizing process whereby the frequency characteristic of the signal is controlled to compensate for any losses caused through a transmission system. Transmitting systems of this kind include, for example, a digital VTR which is arranged to record and reproduced a video signal, as a digital signal, on and from a magnetic tape. The digital VTR is also arranged to perform an equalizing process on the reproduced signal.
The equalizing process is performed by a circuit called an equalizer. The digital VTR of this kind is arranged as follows. FIG. 1 shows in a block diagram the conventional arrangement of a reproduction system of a digital VTR. Referring to FIG. 1, a signal reproduced from a magnetic tape 1 by heads 2A and 2B mounted on a rotary drum D is supplied to a head amplifier 3 through a switch SW. The signal is amplified by about 40 to 50 dB by the amplifier 3 and is then supplied to a reproduction equalizer 5. The switch SW operates, under the control of head switching pulses formed by a head switch pulse forming circuit 14 on the basis of a signal PG which indicates the rotation phase of the rotary drum D, to switch the outputs of the heads 2A and 2B from one over to the other.
The frequency characteristic of the reproduced signal obtained by the magnetic heads 2A and 2B includes, as shown in FIG. 2, a differential characteristic in its low frequency band and an attenuating characteristic in its high frequency band due to losses of varied kinds. The frequency characteristic of the reproduced signal is, therefore, corrected by forming an inverse characteristic combining the resonance characteristic of the head amplifier 3 and the equalizing characteristic of the reproduction equalizer 5. A signal outputted from the reproduction equalizer 5, as shown in FIG. 3 is supplied to a data detecting circuit 10 to be compared with a predetermined threshold value and restored to its original state of digital data. The digital data is demodulated by a demodulator 11. At this time, if the input to the data detecting circuit 10 includes any noise that exceeds the threshold value due to a tape noise, an amplifying noise of the head amplifier 3, or the like, the noise would cause, for example, data which should be "0" to be detected as "1" and thus would result in an error if the data is left as it is.
To avoid this, therefore, an error correcting-and-decoding circuit 12 (hereinafter referred to as an ECC circuit) is arranged to correct errors included in the reproduced signal, by using parity data added at the time of recording, to generate an error flag indicating an uncorrectable error for any data that is uncorrectable and to supply the error flag to a reproduced signal processing circuit 13. The reproduced signal processing circuit 13 is arranged to perform predetermined reproduction processes on the signal obtained from the ECC circuit 12 and to output the processed signal as a reproduced signal.
However, in the conventional digital VTR, the frequency control characteristic of the reproduction equalizer 5 is adjusted only at the time of shipment from a factory and is not arranged to be adjustable by a user after delivery to the user. Therefore, in cases where an optimum control point happens to deviate from what has been set at the time of shipment from the factory due to the kind of the tape and the wear of the head, the errors increase at the time of data detection and, as a result, it becomes impossible to obtain a good reproduced image.
If the user tries to adjust the equalizing characteristic, the high frequency band gain of the equalizer must be varied as shown in FIG. 4, for example, on the basis of a rate of actual errors in the reproduced signal by finding an optimum point where the number of data errors in the reproduced signal can be minimized as shown in FIG. 5. However, in such a case, the characteristic of the equalizer must be varied while a video or audio signal is actually reproduced. The number of uncorrectable data then inevitably increases at the two ends of a characteristic variable range. The increase of uncorrectable data then would affect the reproduced image to lower the quality of the image.
To prevent the quality of the image from deteriorating, if the characteristic variable range is narrowed, it is inevitable to have some spurious optimum point, for example, as represented by a point "b" in FIG. 6. The presence of such a spurious optimum point "b" then would prevent adjustment to a true optimum point "a" shown in FIG. 6.